--mirror-pmOS argument: support multiple repos and no repos

bpo calls pmbootstrap with:

• -mp '' to use no pmOS binary repository (e.g. when running the testsuite). This is currently broken, see trace below.
  • maybe --no-mirror-pmOS would be more elegant?
• multiple -mp arguments to use the final binary repository together with a WIP repository of packages that are built, but not published yet. Looks like this isn't supported either with pmb v3

We'll need to add this again so bpo can use pmb v3.

It looks like the -mp argument is broken in general right now, as it creates a list, but the default is a string to one repository and that's what the rest of the code expects.

[21:35:57] ERROR: expected str, bytes or os.PathLike object, not list
[21:35:57] See also: <https://postmarketos.org/troubleshooting>                                                                                                               
[21:35:57] Traceback (most recent call last):
  File "/home/user/code/build.postmarketos.org/_temp/local_job/pmbootstrap/pmb/__init__.py", line 87, in main          
    run_command(args)          
  File "/home/user/code/build.postmarketos.org/_temp/local_job/pmbootstrap/pmb/commands/__init__.py", line 61, in run_command
    getattr(frontend, args.action)(args)                                                                                                                                      
  File "/home/user/code/build.postmarketos.org/_temp/local_job/pmbootstrap/pmb/helpers/frontend.py", line 134, in build
    built = pmb.build.packages(                                                                                                                                               
            ^^^^^^^^^^^^^^^^^^^                                                        
  File "/home/user/code/build.postmarketos.org/_temp/local_job/pmbootstrap/pmb/build/_package.py", line 394, in packages
    pmb.helpers.repo.update(fallback_arch)                                                                                                                                    
  File "/home/user/code/build.postmarketos.org/_temp/local_job/pmbootstrap/pmb/meta/__init__.py", line 38, in __call__
    self.cache.cache[key] = self.func(*args, **kwargs)
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                        
  File "/home/user/code/build.postmarketos.org/_temp/local_job/pmbootstrap/pmb/helpers/repo.py", line 164, in update
    for url in urls(False):
               ^^^^^^^^^^^
  File "/home/user/code/build.postmarketos.org/_temp/local_job/pmbootstrap/pmb/meta/__init__.py", line 38, in __call__
    self.cache.cache[key] = self.func(*args, **kwargs)
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/code/build.postmarketos.org/_temp/local_job/pmbootstrap/pmb/helpers/repo.py", line 101, in urls
    url = os.path.join(mirror, mirrordir)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen posixpath>", line 76, in join
TypeError: expected str, bytes or os.PathLike object, not list